On 2003 Feburary 15, Don Backer and Glen Langston performed tests
of the GBT at L-Band (1.07 to 1.87 GHz) to determine frequency
ranges of free of Radio Frequency Interference (RFI).
These tests used the L-band receiver with center frequency of 1470 MHz
and the GBT spectrometer as the detector. The spectra were obtained
in dual circular polarization mode.
The observations presented here were
made with the GBT in snow dump position.
The L-band receiver was not in the focus position, so the system temperature
was high, approximately 60 K.
The spectrometer was set up in the 800 MHz wide, single quadrant mode,
with 3 level sampling. For all but the final tests, only the first quadrant
was used. The auto-correllations were obtained in dual polarization mode
with synchronous detection of the Calibration noise diodes being
toggled on and off at a 4 Hz rate.
The IF Rack Optical driver modules 1 and 4 were used for all observations.
The wide, 2360 to 3640 MHz, band pass filters in the IF rack were used.
Converter modules 1 and 5 were used for all tests.
Band Pass Shape Tests
After switching in the different band pass filters, the IF rack was
"balanced". When using the 1.1 to 1.8 GHz band pass filter, the
output of the RCP side had to much power. Even with 31 dB of attenuation
the optical driver module was showing 3 Volts, about 3 times too much
The NOTCH filter is intended to exclude Radar signals in the
band 1.225 to 1.3 GHz.
For the converter module comparision tests, converter modules 1 to 8 were
used. In these tests the spectrometer was set up in the 4 Bank (4 quadrant
mode), with each quadrant measuring the auto-correlations for both
polarizations. There are only small differences in the Band
pass shapes using the different converter modules, analog filters
and spectrometer samplers.
The final plot above shows the system temperature across the band for the
LCP spectra. These data were taken without the notch filter and include
the 1.1 to 1.8 GHz RF filter. The data were calibrated assuming a
constant noise diode value of 1.7 K, which is the appropriate value
for the center of the band, 1.47 GHz.